The present invention relates to drilling tools in general, and more particularly to improvements in drilling tools which can be utilized for enlargement of selected portions of predrilled or basic drill holes in layers of coal or the like.
German Pat. No. 23 31 467 and German Offenlegungsschrift No. 28 56 855 disclose tools which can be used to make conical enlargements or undercuts in the surfaces surrounding relatively short or shallow holes or bores in concrete or a like hard material. The tool has a shaft which carries two bit holders at its forward end which is flattened, and each bit holder constitutes a two-armed lever the front arm of which carries the bit and the rear arm of which has a slot for a guide adapted to pivot the lever whereby the bit moves radially of and away from the axis of the shaft. The guide is held by a stop while the bit holders are caused to move forwardly, i.e., into the hole. The stop which holds the guide is subjected to tensional stresses while the holders move forwardly and thereby pivot so as to move the bits outwardly. During such pivoting of the holders, the bits move away from the pivot axes of the respective holders.
A drawback of the just described conventional tool is that it cannot be used for the making of conical or otherwise configurated enlargements or undercuts in surfaces surrounding relatively deep holes, e.g., holes which are formed in relatively soft material and whose depth is in excess of 1 meter, e.g., between 1.2 and 1.8 m. This is due to the fact that the aforementioned stop is located externally of the hole. Moreover, the material which is removed by the bits of a conventional tool is highly likely to interfere with pivotal movements of the bit holders so that the bits cannot be retracted prior to withdrawal of the tool from the hole upon completion of the enlargement. When the material around the hole is relatively soft, the bits are likely to remove minute particles in the form of powder or dust as well as relatively large fragments which interfere with movements of the bit holders. When in neutral or innermost positions, the bit holders must lie against the flattened front end portion of the shaft in order to allow for extraction of the tool from the hole. The spring which is used in this conventional tool to retract the bits is often too weak to perform its function when the tool is to be withdrawn from the hole, especially after removal of relatively large fragments of material around the hole. Finally, the configuration of surfaces bounding the cam grooves in the rear arms of the bit holders is such that the holders must pivot through relatively large angles which further reduces the likelihood that even a reasonably strong spring could retract the holders preparatory to extraction of the tool from the hole in the event that the hole contains relatively large fragments of removed material.